This invention relates to expandable stents.
Stents are endoprostheses which can be deployed into the lumen of an artery or vein, a common bile duct, the urethra or other body passageway. Stents may be employed in such passageways for many purposes, including expansion of a lumen, maintenance of the lumen after expansion, and repair of a damaged intima or wall surrounding a lumen. With respect to arteries, for example, stents may be used as, or in conjunction with, intralumenal grafts in the maintenance of patency of a lumen following angioplasty. In such cases a stent may be used to prevent restenosis of the dilated vessel, to prevent elastic recoil of the vessel, or to eliminate the danger of occlusion caused by xe2x80x9cflapsxe2x80x9d resulting from intimal tears associated with the angioplasty. In other instances, stents may be used to treat aneurysm, tears, dissections and other continuity faults, as, for example, in the splenic, carotid, iliac and popliteal vessels. By way of further example, it is known to use a stent to maintain the patency of a urethra compressed by an enlarged prostate gland.
In one class of expandable stents commonly referred to as xe2x80x9crolledxe2x80x9d stents, a sheeted material is rolled onto the outer distal circumference of a support member or xe2x80x9ccorexe2x80x9d. The sheeted material is then positioned at a targeted treatment area and expanded. Rolled stents can be characterized according to: (1) the method by which the rolled sheet is maintained in a compressed configuration; and (2) the method by which the sheet is expanded.
Lane, Self Expanding Vascular Endoprosthesis for Aneurysms, U.S. Pat. No. 5,405,379 (Apr. 11, 1995) describes a stent which employs a self expanding sheet. The sheet is forcibly rolled into a compressed configuration, and then inserted into a catheter to maintain the compressed configuration. Expansion takes place by ejecting the sheet from the end of the catheter.
Kreamer, Intraluminal Graft, U.S. Pat. No. 4,740,207 (Apr. 26, 1988) describes a rolled stent in which a sheet of stainless steel is rolled around an angioplasty type balloon. After being introduced into a treatment area, the sheet is expanded by inflating the balloon with a fluid. In this case compression is maintained during the early stages of deployment by the relaxed nature of the sheet in the compressed configuration, i.e., the internal mechanical resistance of the sheet to deformation. Expansion of the sheet, on the other hand, occurs under radial pressure exerted by the expanding balloon.
Sigwart, Intravascular Stent, U.S. Pat. No. 5,443,500 (Aug. 22, 1995) describes a stent in which a flat sheet is perforated to form a reticulated or lattice type structure having a ratcheting locking mechanism. Compression in stents according to the Sigwart patent are maintained by a holding wire or adhesive, and the sheet is contemplated to be expanded under the influence of an angioplasty balloon.
Sigwart also describes another stent comprising an elastic stainless steel mesh. The diameter of the mesh is slightly larger than the normal inner diameter of the vessel to be treated, so that the mesh can exert a residual radial pressure on the arterial wall after being implanted. Before being introduced into a patient""s blood vessel the stent is reduced in diameter. The reduced diameter is maintained while advancing the stent into a target treatment area by an outer sleeve. Once the device is implanted, the stent is deployed by withdrawal of the outer sleeve. In this instance, compression is thus maintained by the outer sleeve, and expansion is achieved by removal of the outer sleeve.
Alfidi and Cross, Vessel Implantable Appliance and Method of Implanting It, U.S. Pat. No. 3,868,956 (Mar. 4, 1975) describes a stent which utilizes a recovery alloy such as nitinol. In such stents an initial expanded configuration is permanently set into the alloy by heating the material to a relatively high temperature while the alloy is maintained in the expanded configuration. The alloy is then cooled and deformed to a compressed configuration. The compressed configuration is retained at room temperature, but recovers to the expanded configuration when reheated to a transition temperature. Here, compression is maintained during the early stages of deployment by the internal mechanical resistance of the alloy against deformation, and the sheet is expanded under the influence of heat.
These and all other known teachings reflect the accepted wisdom that rolled stents are to be maintained in their compressed configurations by the operation of static forces (e.g., biasing produced by the internal mechanical resistance of the sheet to deformation, presence of holding wires, outer sleeves and so forth), while expansion of the sheeted materials is to be produced by application of a dynamic force (e.g., radial pressure exerted by an expanding balloon, application of heat, removal of a holding wire or sheath, and so forth). While such strategies undoubtedly have their benefits, it is useful to have stents which operate outside of these accepted constraints.
Where the stent is to be deployed in very small vessels of the body, such as the arteries in the brain, the size of the stents is quite small, and the material used for the stents is on the order of 0.0001-0.0004 inches thick. The small size and extreme thinness of the stent material makes it difficult to deploy the stent using the typical push-pull type deployment mechanisms generally used for stents. The frictional force exerted on the stent by the catheter sheaths and cores as they slide over the stent often tears the stent. In our co-pending U.S. patent application 08/762,110, filed Dec. 9, 1996, we provide a number of devices that do not require any sliding movement of the stent or catheter sheath relative to each other. The devices described below provide additional mechanisms and methods for deploying stents while minimizing the frictional forces operating between the stents and the catheters used for their insertion.
Stents for intra-cranial use and methods for using these stents are described in detail below. The physical characteristics of prior art balloon expandable stents and self expanding stents make them clearly unsuitable for intra-cranial use, because of their delivery profile, their lack of flexibility and their tendency to temporarily occlude the vessel during deployment. They have not been proposed for intra-cranial use. Palmaz stents, Palmaz-Schatz(trademark) stents, Wallstents, Cragg stents, Strecker stents and Gianturco stents and other stents are too rigid to allow placement in the cerebral blood vessels, some require a balloon for deployment, and all are too open to occlude or prevent blood flow into an aneurysm. Presented below are several embodiments of stents suitable for intra-cranial use, along with methods for using these stents to treat intra-cranial vascular disease.
The self expanding rolled sheet stent is suitable for use in the intra-cranial arteries. The rolled sheet is made of Elgiloy(trademark), nitinol, stainless steel, plastic or other suitable material, and is imparted with resilience to urge outward expansion of the roll to bring the rolled stent into contact with the inner wall of a diseased artery. The rolled sheet is adapted for easy insertion and non-deforming radial flexibility to facilitate tracking along the tortuous insertion pathways into the brain. In some embodiments, as much of the material of the stent is removed as is consistent with eventual creation of a solid walled stent upon unrolling of the stent within the blood vessel. The unrolled stent may be two or more layers of Elgiloy(trademark), thus providing radial strength for the stent and creating at least a slight compliance mismatch between the stent and the blood vessel, thereby creating a seal between the stent and the blood vessel wall. For placement, the stent is tightly rolled upon or captured within the distal tip of an insertion catheter. The release mechanism is extremely low profile (3 Fr or less), and permits holding the rolled stent in a tight roll during insertion and permits atraumatic release when in the proximity of the site of arterial disease, without completely occluding the vessel with the deployment catheter. The stent can be placed in the intra-cranial blood vessels (arteries and veins) of a patient to accomplish immediate and complete isolation of an aneurysm and side branches from the circulatory system. The stent can be placed so as to partially occlude or modify blood flow into an aneurysm. The stent can be placed so as to allow for injection of coils (GDC coils or Gianturco coils) or embolic material into an aneurysm and prevent wash-out of the coils or embolic material. The stent may be placed across a target site such as an aneurysm neck, origin of a fistula, or branch blood vessels feeding a tumor in order to redirect the flow of blood away from the target. It can be used as a stand alone device which is left in the intra-cranial artery permanently, or it may be used as a temporary device which allows for immediate stabilization of a patient undergoing rupture of a blood vessel an aneurysm or awaiting open skull surgery for clipping or resection of an aneurysm. The stent can be used for stabilization and isolation of a vascular defect during surgery of the vascular defect. Another advantage of this type of stent is that it can be wound down should repositioning be required prior to full release. It is possible to rewind and reposition or remove the device using a crooked rotating wire or other grasping tools.
The present invention is directed to expandable stents having a sheeted material configurable in both a compacted configuration and an expanded configuration, in which the compacted configuration during deployment is maintained at least in part by operation of a dynamic force, and expansion to the expanded configuration occurs at least in part by removal of the dynamic force.
In preferred embodiments, a biocompatible sheeted material is wrapped around a retaining wire to produce the compacted configuration, and the retaining wire is rotated to maintain the compacted configuration during insertion of the stent into a patient. The rotation is continued while the stent positioned, and then the rotation is stopped or slowed to permit expansion of the sheet. Counter rotation may be used to enhance expansion, if desired.